slack-welcomer: fix ingress API, update welcome message, fix container name

[Champbreed]

This PR migrates cluster/ingress.yaml from the deprecated extensions/v1beta1 to networking.k8s.io/v1 with updated backend syntax, replaces all broken git.k8s.io links in message.yaml with working GitHub paths while adding the Slack Guidelines and updating channels, also fixes a container naming typo in deployment.yaml where the container was labeled slack-moderator instead of slack-welcomer.

/sig contributor-experience
/cc @jberkus @mrbobbytables @BenTheElder @Priyankasaggu11929

Fixes #76

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[k8s-ci-robot]

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[Champbreed]

/ok-to-test

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/ok-to-test